1. Field of the Invention
This invention relates to a semiconductor device, semiconductor laser, their manufacturing methods and etching methods, and more particularly, to a semiconductor device using nitride semiconductors and requiring selective processing for a current confining structure, for example. The invention also relates to, in particular, a high-performance semiconductor laser controlled in current confinement and transverse mode, and a manufacturing method thereof.
2. Related Background Art
Group III nitride semiconductor materials, which enable realization of maximum band gap energies among III-V compound semiconductor materials and can make hetero junctions, are remarked as hopeful materials of semiconductor lasers and light emitting diodes for emission of short-wavelengths, or high-speed, high-output electronic devices. For typical devices using group III nitride semiconductor materials, thin-film forming techniques using metal-organic chemical vapor deposition (MOCVD) and epitaxial growth such as molecular beam epitaxy (MBE) are often used. In case of electronic devices having hetero junctions, such as semiconductor lasers, light emitting diodes, etc., those thin-film growth techniques are used to form a plurality of nitride mixed crystal thin film layers of group III elements different in composition ratio and use differences in band gap energy among these layers to confine light or electrons.
Such nitride mixed crystal thin film layers are typically formed on various base bodies. Upon their epitaxial growth, composition of a mixed crystal thin film layer grown under a fixed growth condition, i.e. composition of a mixed crystal thin film layer grown in a single process of crystal growth, was usually uniform over the entire surface. Usually, therefore, physical properties of the mixed crystal, such as bandgap energy, refractive index, conductivity, specific resistance, and soon, were uniform over the entire surface of the thin film layer formed on the base body. Although there is a report about generation of non-uniformity, which reports fine deposition regions different in composition are formed in an InGaN thin film, from a macro-scale viewpoint, those physical properties are not but ones that should be regarded to be substantially uniform throughout the region having formed the thin film layer on the base body. Additionally, there is a report also regarding group III nitride semiconductors that a so-called “superlattice”, made by periodically forming a plurality of very thin films of a thickness several to tens of times an atomic layer, has been made. Here again, those physical properties of the entirety of the superlattice layer are not but ones that should be regarded to be substantially uniform over the entire surface of the thin film layer on the base body.
Thus it has been considered that composition and physical properties of any nitride mixed crystal thin film layer made in a single process of crystal growth are inevitably uniform over the entire surface of the thin film layer. Therefore, in order to intentionally vary physical properties of the nitride mixed crystal thin film layer in the surface direction of the base body in a semiconductor laser, light emitting diode, electronic device, or the like, it has been necessary to carry out a plurality of epitaxial growth steps and etching steps, as well as additional complicated steps for positional alignment.
FIG. 12 is a cross-sectional view that shows configuration of a conventional semiconductor laser using nitride mixed crystal thin film layers. The laser of FIG. 12 includes an n-type GaN contact layer 912, n-type AlGaN cladding layer 914, InGaN quantum well active layer 916, p-type AlGaN cladding layer 918, and p-type GaN contact layer 920, which are thin film layers uniform in the surface direction, formed on a surface of a sapphire substrate 910 as a base body. The p-type cladding layer 918 is ridge-shaped to enhance the optical guide efficiency. For current confinement, the laser further includes an insulating film 930 having an opening above the ridge of the p-type cladding layer 918, and through this opening, a p-side electrode 950 is formed. Connected to the n-type contact layer 912 is an n-side electrode 940.
The semiconductor laser shown in FIG. 12 needs a complicated process including selective etching of the p-type cladding layer and others for making the waveguide, current confinement or electrode contact, formation of the insulating film 930, formation of the p-side electrode 950 and n-side electrode 940, and so on. It therefore involves the problems that the production yield is low and the productivity necessary for reducing the cost is low. Additionally, there is the problem that damage to crystals during etching and other process deteriorate the initial characteristics and reliability of the device.
As reviewed above, conventional techniques could only obtain uniform physical properties of any nitride mixed crystal thin film layer formed on a base body. So, for fabricating a semiconductor laser, light emitting diode, electronic device, or the like, the conventional techniques had to use processing techniques requiring a plurality of epitaxial growth and complicated positional alignment in order to vary physical properties such as bandgap energy, refractive index, conductivity and specific resistance along the horizontal surface of the base body and for hereby eliciting functions. And this invited the problems that the production yield was low, productivity necessary for reducing the cost was low, or damage to crystals during the use of those processing techniques deteriorated the initial properties and reliability of the device.
On the other hand, apart from those problems, semiconductor lasers using nitride semiconductors had need of a technique that could reliably stop etching at a predetermined etching depth.
That is, blue semiconductor lasers using nitride semiconductors like InAlGaN, which have short wavelengths and can therefore make small beam diameters, are recently looked for as light sources for high-density information processing with optical disks, for example. For application to optical disc systems, for example, it is necessary to converge emanating beams of semiconductor lasers to minimum spots, and basic transverse mode oscillation is indispensable.
A number of devices with conventional ridge structures have been reported as nitride semiconductor lasers. Ridge structures, however, are characterized in that the difference in effective refractive index between the ridge portion important for transverse mode control and the exterior of the ridge largely depends on the etching depth. For years, dry etching represented by reactive ion etching (RIE) and reactive ion beam etching (RIBE) has been widely used in the etching process for making the ridge. However, regarding dry etching of nitride semiconductors, there is not yet established any technique, such as selective etching method, capable of stopping the etching at a target etching depth, and the etching depth is controlled by adjusting the etching time or by monitoring the progress of the etching through a laser interferometer, for example. With these control methods of the etching depth, however, it is difficult to stop the etching at the interface with the underlying layer or stop the etching so as to keep a desired thickness over the entire wafer surface, and sufficient control of the etching depth is impossible.
Thus, the conventional etching techniques cannot control the etching depth sufficiently. Additionally, since ridge structures are affected by the thickness profile of the film by crystal growth, etching depth profile, and so forth, it was difficult to fabricate devices controlled in basic transverse mode with a good yield. That is, in InAlGaN semiconductor lasers having conventional ridge structures, their structures themselves invite large influences to their characteristics from process accuracy and unevenness. Therefore, it was difficult to fabricate lasers for continuous oscillation in the basic transverse mode with a good yield.